Bridge-builders Mabey & Johnson, the firm owned by one of Britain's wealthiest families, which has been under prolonged investigation by the Serious Fraud Office, is to plead guilty to bribery offences and breaching UN sanctions.

The firm is expected to appear at Westminster magistrates court on Friday in a ground-breaking SFO prosecution, according to sources close to the firm's lawyers.

A substantial fine is expected, following lengthy confidential negotiations with the auhtorities and a pioneering deal. This is the first time the SFO has successfully negotiated the equivalent of a US plea-bargain with a firm accused of overseas corruption.

The corruption charges relate to contracts obtained by the company in Jamaica and Ghana. The sanctions charges relate to accusations in the UN Volcker report that Mabey paid a $200,000 kickback to the Saddam regime in Iraq between 2001 and 2003 in return for a $3.6m bridge contract.

Questions about Mabey's behaviour were first raised by the Guardian in 2005, when the paper revealed large commission payments made for bridge contracts in the Philippines. Loans were backed by the British taxpayer's official export agency, the ECGD.

The firm were also accused by campaigners of making payments to the ruling party in Papua New Guinea.

Neither of those allegations led to criminal charges. But, following a dispute with a former Mabey executive, detailed evidence subsequently emerged in a civil case of improper commission payments being made in Jamaica.